The procedure for permanently altering the contour of keratin fiber in hair is a two stage process. Cleavage of the cystine disulfide bond is the first step (i.e., reduction) while reforming these bonds, usually in a new configuration, by treating the reduced fiber with an oxidizing agent (i.e., neutralizer) is the second step.
Permanent waving of human hair is achieved by winding the fiber onto perm rods during this reduction and neutralization procedure. If performed correctly, this action will place the fiber under some stress. A waving lotion, usually consisting of a water soluble mercaptan (i.e., salts and esters of thioglycolic acid, cysteine, cysteamine, etc.), is then applied to the hair. The pH of this lotion normally can vary from pH 6.5 to pH 9.9. Under these conditions, the cystine disulfide bonds are cleaved (i.e., reduced) and fiber stress is removed by molecular rearrangement of the polymeric keratin structure. Once the new configuration is achieved, the fiber is treated with an oxidizing agent such as hydrogen peroxide, sodium bromate, or sodium chlorite, to "lock" the fiber into the newly formed configuration.
Reducing reagents can also be used to straighten curly hair; that is, the curl stress is removed by reduction of the hair followed by brushing and/or combing until the curl is relaxed and the fiber is in the straight configuration. The fiber is then neutralized to maintain the new "curl-less" shape. Sulfites and bisulfites, operating through a different chemical sequence of reactions, have also been utilized to cleave keratin disulfide bonds and permanently alter the fiber structure.
Low molecular weight mercaptans are foul smelling organic molecules that often contain malodorous impurities. When hair fibers are treated with these chemicals during the reduction stage, some of the mercaptans, together with impurities and byproducts formed by the various chemical reactions occurring are trapped into the porous keratin fiber structure. Release of these residual malodorous chemicals is responsible for the post-perm obnoxious odors. In general, the more porous the hair fiber, the more retentive the malodors. These malodors often linger and remain in the fiber even after several shampoo treatments.